Although light emitting diodes (LEDs) hold great promise for application ranging from telecommunications to general illumination, the cost per-lumen still hinders LED's penetration of the markets. Currently, the lighting market is dominated by compact fluorescent lamp (CFL). The cost per-lumen for LED luminaires must rapidly decreases to compete with CFLs.
One way to realize the price-reduction objectives for LED lights without significantly changing the device manufacturing cost is to increase the injection current density, for example by a factor of 2 to 4, from an order of tens of A/cm2 to hundreds of A/cm2. However, increasing the light-power output of devices through increasing the drive-current of LEDs could lead to two problems due to increased heat generation. One is the effect of “efficiency droop” and the other one is the effect of “thermal runaway”. If the heat cannot be dissipated properly, the higher junction temperature will lead to lower EQE (external quantum efficiency) of the LED device, which will lead to an even higher temperature and eventually lead the LED devices to thermal failure. Therefore, the thermal management of the LEDs is a key issue to decreasing the cost of LED lights without significantly changing the manufacturing cost of LED chips. Additionally, keeping the junction temperature as low as possible is also beneficial to the lifetime of LEDs. In summary, LED thermal management is critical to lowering junction temperature, increasing light power output and lifetime.
Heat transfer process follows the following rule:Q=hAΔTwhere Q is the heat transfer power (W), h is the heat transfer coefficient (W/(m2·K)), A is the area of thermal pass, and ΔT is the temperature gradient or difference. The heat transfer coefficients of different heat transfer mechanisms are different. Because of the considerable difference of h between different heat transfer mechanisms, it is necessary to evenly spread the heat to different thermal pass area to achieve an effective cooling system.
The thermal model of a common LED system is depicted in FIG. 1. The system thermal resistance of the LED device can be divided into three categories or stages: Rinner, Rinter, and Rexter. Rinner includes the thermal resistance of the LED chip (Rchip), the thermal resistance of the sub-mount bonding (Rbonding), and the thermal resistance of the substance of the substrate and back solder. Rinner is mainly determined by the chip design and the materials used in fabricating the chip. Rinter refers to the thermal resistance derived from the printed circuit board (PCB) and thermal interface materials (TIM). Rexter relates to the thermal resistance from the TIM to the atmosphere.
FIG. 3 shows a LED light structure employing a conventional cooling mechanism to dissipate heat from the LED chips to the environment. The LED chips 301, along with necessary PCB and TIM, are mounted on a surface of a cooling fin structure 303 and enclosed in a cover 302. The fin structure 303 is formed of multiple solid plates made of metal. The LED light also has a connector 305 for affixing it to a conventional lighting fixture, and a power unit 304 containing circuitry for driving the LED chips.
Comparing with the typical values of Rinner and Rinter, Rexter based on passive heat sink according to conventional technologies often cannot satisfy the application demands for LEDs driven by high injection currents. The thermal resistance of passive heat sink is caused by its poor heat match or spreading. Phase change cooling systems, which conduct heat away through phase change at a high temperature region and reverse phase change at a low temperature region, can improve the heat spreading significantly.
1-D heat pipe and 2-D vapor chamber are two widely used phase change cooling systems. Both of them have been applied in thermal management of LEDs, for example, as described in Lan Kim et al., Thermal analysis of LED array system with heat pipe, Thermochimica Acta, 455, 21-25 (2007) (“Kim et al. 2007”); and H.-S. Huang et al., Experimental Investigation of Vapor Chamber Module Applied to High-Power Light-Emitting Diodes, Experimental Heat Transfer, 22, 26 (2009) (“Huang et al. 2009”). In such systems, the heat pipe and vapor chamber function as a heat spreader between the heat source and the lower temperature region. As shown in FIG. 2, the heat pipe and vapor chamber still need to be coupled with heat sink in actual application. A heat pipe spreads the heat from a heat source to a heat sink through a one-dimensional phase change heat transfer structure (See FIG. 2(a)). The typical thermal resistance of a heat pipe coupled with a heat sink is about 5 K/W (see Kim et al. 2007). A vapor chamber spreads the heat through a two-dimension phase change heat transfer structure (See FIG. 1(b)). The typical thermal resistance of a 2-D vapor chamber coupled with a heat sink is 3.2-4.9 K/W (see Huang et al. 2009).